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Common Causes of Fish Kills


There’s nothing appealing about a bunch of dead fish floating past your favorite swimming spot.

And it’s not comforting to see fish belly up where you planned to cast a line to catch your supper or have your family swim.

Recent fish kills in the upper reaches of the Brazos River have left many people concerned about the cause and potential fallout from these kills.

It’s important to understand there are many different reasons a fish kill can occur.

A fish kill refers to a localized die-off of one species or multiple species of fish.

So, what causes fish kills? There are actually a number of reasons large numbers of fish might die, but let’s look at a few:


  • Dissolved oxygen:

    Low dissolved oxygen levels are the most common cause of fish kills in Texas, according to the Texas Parks and Wildlife Department. There are two main developments that can lead to low dissolved oxygen in water bodies.
    1. The amount of dissolved oxygen that water can hold is directly related to temperature. Water can hold less oxygen when temperatures are high. After prolonged heat spells, shallow areas of rivers or streams are not capable of holding enough oxygen to support large numbers of fish. Key indicators of dissolved oxygen events include:
      a. Large fish die-off, while small fish are still alive
      b. Fish are seen gulping for air at the surface of the water
      c. Dead fish are often white in color
    2. High concentrations of algae, most commonly green or blue-green algae, in a water body can also lead to low dissolved oxygen. During the day, the algae release high concentrations of oxygen into the water through the process of photosynthesis, making these areas appealing to fish, especially during warm weather. However, once the sun goes down and the algae are no longer able to photosynthesize, they move to a process called respiration, where the algae then absorbs the oxygen it produced during the day to help it survive the night. This process leaves little oxygen in the water for fish that had been attracted to the area earlier in the day. In addition to the three indicators above, low dissolved oxygen events caused by algae also occur early in the morning.

  • Water Temperature:

    Also common in the Brazos basin are water temperature related fish kills caused by a sudden drop in water temperatures as cold fronts move rapidly into an area. This type of fish kill is more selective to species that cannot tolerate colder temperatures or species that are stressed by sudden changes in temperature. Shad are one of the species most frequently affected in the Brazos basin by rapid temperature changes.

  • Harmful Algal Blooms:

    There are many types of algae that occur naturally in all surface water and are a base organism in the aquatic food pyramid. While most algae are not problematic, some types of algae have the potential to produce toxins that are harmful to aquatic organisms, and in some cases, humans, pets, livestock and wildlife.
    1. Golden Alga
      Golden alga or Prymnesium parvum, is one type of microscopic alga that typically occurs in brackish or salty water. When this alga blooms, it can produce toxins that are lethal to fish, mussels and clams.
      There is no evidence the toxins produced by golden alga are harmful to humans, livestock or wildlife. It characteristically appears as brownish or tea-colored water. The alga tends to expand during colder months. In 2003 and 2005, the freshwater organism nearly devastated some fish populations.
      While golden alga is frequently present in the waters of the Brazos basin in low concentrations, it is most likely to bloom, become toxic, and cause fish kills in cold weather. A golden alga fish kill may last for days, weeks or months, and may affect whole water bodies or isolated portions of water bodies.
      With the historically high frequency of wintertime golden algal fish kills, BRA aquatic scientists partner with the Texas Parks and Wildlife Department to take monthly samples for early detection of potential golden alga issues on Possum Kingdom Lake and Lake Granbury.
      Indicators of toxic golden algal blooms:
      • Water appears to be tea-colored
      • Small fish appear to be impacted first, with larger fish impacted as the duration of the toxic event continues, expands in area, or increases in intensity. Golden alga will affect all species of fish.
      • Fish may be gulping for air at the waters' surface because the toxin attacks their gills and the gills no longer function appropriately.
      • Fish swimming in irregular patterns or manner, often appear to be intoxicated.
      • Hemorrhaging is visible in the fish. It is first noticeable in the gills, head and base of the fins, then moves into the tissue giving the fish a pink to red appearance.
    2. Blue-Green Algae
      Most blue-green algae, also called cyanobacteria, are not known to produce toxins; but, there are two species, Anabaena and Microcystis, known to occur in Texas that can produce toxins. Unlike golden alga, these two blue-green algae species not only cause fish kills when toxic blooms occur, but their toxins can be harmful to humans, pets, livestock and wildlife.
      Fortunately, there have not been any recorded toxic blue-green alga blooms recorded in the Brazos basin in more than 25 years.
    3. Red Tide
      Red tide is an algal bloom that is caused by the algae Karenia Brevis. Red tide events occur mostly in the Gulf of Mexico and associated estuaries, the area where freshwater from rivers mixes with the saltwater in the Gulf. While rare in Texas, red tide events have occurred in Galveston Bay near the Brazos basin estuary. Red tide blooms occur mostly in late summer or early fall and cause the water to appear to be red to brown in color.
      Toxic red tide events not only result in fish kills but are also harmful to humans, pets, livestock and wildlife. Additionally, the toxin produced during red tide events is heat-stable and not neutralized during the cooking process, so it is best not to fish in areas where red tide events are occurring.

  • Toxins, pesticides, fertilizers and chemicals:

    People are sometimes the result of fish kills. If there is a spill near a waterway of different types of chemicals, depending on the amount, fish can die. Environmental spills should also be reported.

  • Diseases, Virus, Bacteria and Parasites

    Just like with people, fish are subject to illness. Illness is naturally occurring and often only affects one species of fish at a time. In addition to only one species being affected, other indicators of an illness-related fish kill include sores, missing scales or lack of slime, abnormal growths, visible parasites, and abnormal behavior.

If you see a fish kill on the Brazos River or anywhere in the state, call the Texas Parks and Wildlife Department’s 24-hour communication center at (512) 389-4848 or email le.communications@tpwd.texas.gov. Prompt notification is key to successfully determining the cause of a fish kill.

It is helpful to the TPWD Kills and Spills Team to provide as much information as possible such as the location, date, and time you noticed the fish kill. If possible, also note if there is an obvious change in the water's color, clarity or if there is any odor.

If you’re familiar with fish, try to estimate the number, size and species you see, or if they are all simply very small or large. You might also provide observations on recent weather conditions and behavior of the fish. Are they all dead or are some still alive? You might also note changes, if any, to plants or other animals you may see that are affected.

When there is a report of dead fish in Brazos basin reservoirs or rivers, the BRA’s environmental team will assist the Texas Parks and Wildlife Department’s Kills and Spills Team, document the event and investigate the cause.

Most of the time, dead fish will not be removed from waterways. As part of the natural life cycle, the fish will usually be consumed by other wildlife or will decay through natural processes.

No matter the cause, there are also good standard practices any time dead fish are in your area. Good practices include not swimming near dead or dying fish and not eating dead or dying fish. Only eat fish that appear healthy at the time they are caught.